Beamforming systems in wireless networks, e.g., beamforming transmitters, beamforming receivers, etc., provide directional signal control by combining signals transmitted or received by two or more antenna elements of an antenna array such that signals at particular angles experience constructive interference, while other signals experience destructive interference. Such directional control provides improved coverage and less interference in the wireless network. For simplicity, the following refers to beamforming transmitters. It will be appreciated, however, that the problems and solutions discussed herein apply to any element utilizing beamforming or phase control, including beamforming receivers.
The directional control of beamforming transmitters may be achieved by controlling the phase and relative amplitude of the signal applied to each antenna element. Thus, the performance of the beamforming transmitter is inextricably tied to the accuracy of the phase control of each antenna element. Some systems implement local oscillator beamforming, which involves phase shifting the local oscillator (LO) signal for each antenna element to achieve the desired phase shift for the corresponding antenna element. Such phase control of the LO signal requires good control of the static phase differences between the separate transmitters.
The static phase differences between the transmitters, however, can be difficult to predict. For example, an integrated direct upconversion radio typically produces the upconversion mixing signal by dividing the LO signal with, e.g., a digital quadrature frequency divider. Such a divider may start up in any one of its possible internal states, where each state is associated with a different phase. This is a problem for beamforming transmitters, particularly in time-division duplex (TDD) systems, where each transmitter is powered down between transmission slots. Because each antenna element has its own transmitter, the resulting radiation pattern will constantly change between the transmission slots because the initial phase for each antenna element is unknown every time the transmitter is powered back on. This same problem also exists for TDD beamforming receivers, and transmitters and/or receivers in Multiple Input, Multiple Output (MIMO) systems. Thus, there remains a need for accounting for such start-up phase variations when controlling the phase of the antenna signals.